<!DOCTYPE html>
<body>
  <div id="source" style="font-size: 10px; -moz-column-gap: 2ch">
    <span id="test"
          style="background: green; display: inline-block; height: 30px;
                 padding: 0 2px">
    </span>
    <script>
      document.getElementById("test").style.width =
        document.defaultView.getComputedStyle(document.getElementById("source"),
                                              "").MozColumnGap;
    </script>
  </div>
</body>
